Preparation Of ZrB₂@ZrO₂ Powder By In Situ Passivation Method And The Oxidation Resistance Of Its Sintered Composite

As a kind of ultra-high temperature ceramics(UHTCs),ZrB₂ ceramic has excellent application potential in the ultra-high temperature field.It has excellent comprehensive properties,such as high melting point,high conductivity,high thermal conductivity and low density.However,its low fracture toughness and poor oxidation resistance limit its wide application.In order to simultaneously improve the toughness and high temperature oxidation resistance of ZrB₂ ceramics,the ZrO₂ was introduced into the toughening system of ZrB₂-ZrO₂ in the form of coating.To investigate the contribution of coating structure to mechanical properties and oxidation resistance,the in situ chemical passivation technology was used to prepare the ZrB₂-ZrO₂ composite powder with a uniform coating thickness and 100%coverage.A series of dense ZrB₂-ZrO₂ ceramics were prepared by plasma activated sintering(PAS)process.The electrochemical passivation of ZrB₂ in Na OH solution was studied,the passivation mechanism was revealed,and the passivation kinetic equation was established.These laid a foundation for the preparation of ZrB₂@ZrO₂ composite powder.The results showed that ZrB₂ ceramic could achieve electrochemical passivation in Na OH solution.Its passivation potential range is–0.8?0.8 V(SHE).The passivation film mainly consists of amorphous ZrO₂ and a small amount of Zr(OH)₄.The thickness increases uniformly with time,and the grown speed is only determined by Na OH concentration and temperature.The in situ chemical passivation process of ZrB₂@ZrO₂ powder was investigated.The composite powder can be prepared in 1 M Na OH solution.ZrB₂ powder began in situ passivation reaction to form the amorphous ZrO₂.The subsequent hydrothermal crystallization process includes the in-situ crystallization of amorphous ZrO₂ directly to t-ZrO₂ and the formation of m-ZrO₂ by dissolution-nucleation growth recrystallization of amorphous ZrO₂.Thus a double shell structure with the outermost m-ZrO₂ and inner t-ZrO₂ coexisting with the amorphous ZrO₂ is formed.Under the conditions of 140°C and 2 M H₂O₂ solution,a series of composite powders with different zirconia content and uniform thickness can be obtained by adjusting reaction time.The above composite powder was used to prepared the ZrB₂@ZrO₂ ceramics by PAS process;And its sintering densification behavior was studied;The densification mechanism was revealed;The microstructure evolution was investigated;The formation of the coating structure in the ceramics was revealed;The relationship between the microstructure and the mechanical properties was studied by contrast the ZrB₂-ZrO₂ ceramics without the coating structure.ZrB₂@ZrO₂ powder can achieve approximately 95%of theoretical density at 1700°C;However,sintering pressure and holding time do not contribute much to densification.The ZrO₂ coating of the powder prevents direct contact between micron-sized ZrB₂ powders during sintering.Thus,the main sintering process is transformed to be the sintering of ZrO₂.And the sintering between ZrB₂ and ZrO₂ requires a higher sintering temperatures(>1500°C).The overburning phenomenon of the ZrO₂ phase occurs when ZrO₂ content is too high.The composite powder has the best sintering performance when ZrO₂ content is 20 wt%.The dense ZrB₂@ZrO₂ ceramic structure helps to improve its mechanical properties.And the introduction of 3 mol%Y₂O₃ can stabilize all m-ZrO₂ into t-ZrO₂.The large existence of t-ZrO₂ produces the martensitic toughening effect,which further promotes the fracture toughness of the ZrB₂@ZrO₂ ceramics.ZrB₂@ZrO₂(3Y)ceramic with high toughness and high density was used as the research object.The effect of coating content on the oxidation resistance was studied;The oxidation resistance at different oxidation temperatures was evaluated and analyzed;The contribution of ZrB₂@ZrO₂ coating structure to the high temperature oxidation resistance was investigated.The synergistic antioxidant mechanism of SiC addition and ZrB₂@ZrO₂ coating structure was investigated.The results show that ZrB₂@ZrO₂(3Y)ceramic has the obvious segmentation effect and extrusion effect during oxidation.The segmentation effect is the destruction of the continuity of ZrB₂phase by the original ZrO₂ phase in the ceramic,which makes the internal liquid B₂O₃less continuous than pure ZrB₂ during the oxidation process.The segmentation effect makes its antioxidant performance lower than that of pure ZrB₂ when it is lower than1400°C.The extrusion effect is that the original dense ZrO₂ phase of the ceramic interacts with the oxidized expanded ZrB₂ phase,causing the results that the liquid phase B₂O₃ uniformly fill the inside of the oxide layer and even flow out of the surface.Extrusion ensures the good oxidation resistance at 1400°C.When ZrO₂ coating content was 20 wt%,ZrB₂@ZrO₂(3Y)ceramic realized the balance of segmentation effect and extrusion effect,and thus had the best oxidation resistance at 1400°C.The addition of a small amount of SiC in the ZrB₂@ZrO₂(3Y)ceramic can ensure its original extrusion effect,but also can synergically combine the contribution of SiC.However,excessive introduction of SiC(>8 vol%)will consume more Y₂O₃,resulting in cracks in the oxide layer.When the content of SiC is 8 vol%,ZrB₂@ZrO₂/SiC ceramic have the best oxidation resistance at high temperature.